From: David Gagnon <dgagnon@siunik.com>
To: Stephan Szabo <sszabo@megazone.bigpanda.com>
Cc: pgsql-performance@postgresql.org
Subject: Re: Why the planner is not using the INDEX .
Date: Mon, 04 Jul 2005 20:29:50 -0400
Message-ID: <42C9D47E.7090103@siunik.com> (raw)
In-Reply-To: <20050704132446.O14579@megazone.bigpanda.com>
References: <20050630222451.GA17628@calvin.surfutopia.net>
<42C473BA.9090305@commandprompt.com>
<20050630235821.GA2163@calvin.surfutopia.net>
<42C994BD.3000909@siunik.com>
<20050704132446.O14579@megazone.bigpanda.com>
Thanks .. I miss that FK don't create indexed ... since Primary key
implicitly does ...
I'm a bit surprised of that behavior thought, since it means that if we
delete a row from table A all tables (B,C,D) with FK pointing to this
table (A) must be scanned.
If there is no index on those tables it means we gone do all Sequantial
scans. Than can cause significant performance problem!!!.
Is there a reason why implicit index aren't created when FK are
declared. I looked into the documentation and I haven't found a way to
tell postgresql to automatically create an index when creating la FK.
Does it means I need to manage it EXPLICITLY with create index statement
? Is there another way ?
Thanks for you help that simple answer will solve a lot of performance
problem I have ...
/David
>On Mon, 4 Jul 2005, David Gagnon wrote:
>
>
>
>> If you can just help my understanding the choice of the planner.
>>
>>Here is the Query:
>> explain analyse SELECT IRNUM FROM IR
>> INNER JOIN IT ON IT.ITIRNUM = ANY ('{1000, 2000}') AND
>>IT.ITYPNUM = 'M' AND IR.IRYPNUM = IT.ITYPNUM AND IR.IRNUM = IT.ITIRNUM
>> WHERE IRNUM = ANY ('{1000, 2000}') and IRYPNUM = 'M'
>>
>>Here is the Query plan:
>>
>>QUERY PLAN
>>
>>Hash Join (cost=1142.47..5581.75 rows=87 width=4) (actual
>>time=125.000..203.000 rows=2 loops=1)
>> Hash Cond: ("outer".itirnum = "inner".irnum)
>> -> Seq Scan on it (cost=0.00..3093.45 rows=31646 width=9) (actual
>>time=0.000..78.000 rows=2 loops=1)
>> Filter: ((itirnum = ANY ('{1000,2000}'::integer[])) AND
>>((itypnum)::text = 'M'::text))
>>
>> -> Hash (cost=1142.09..1142.09 rows=151 width=37) (actual
>>time=125.000..125.000 rows=0 loops=1)
>> -> Index Scan using ir_pk on ir (cost=0.00..1142.09 rows=151
>>width=37) (actual time=0.000..125.000 rows=2 loops=1)
>> Index Cond: ((irypnum)::text = 'M'::text)
>> Filter: (irnum = ANY ('{1000,2000}'::integer[]))
>>Total runtime: 203.000 ms
>>
>>
>
>
>
>> I don't understand why the planner do a Seq Scan (Seq Scan on table
>>IT ..) instead of passing by the followin index:
>> ALTER TABLE IT ADD CONSTRAINT IT_IR_FK foreign key (ITYPNUM,ITIRNUM)
>>references IR (IRYPNUM, IRNUM) ON UPDATE CASCADE;
>>
>>
>
>That doesn't create an index on IT. Primary keys (and unique constraints)
>create indexes, but not foreign keys. Did you also create an index on
>those fields?
>
>Also it looks like it's way overestimating the number of rows that
>condition would succeed for. You might consider raising the statistics
>targets on those columns and reanalyzing.
>
>
>
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To: pgsql-performance@postgresql.org
Cc: dgagnon@siunik.com, sszabo@megazone.bigpanda.com
Subject: Re: Why the planner is not using the INDEX .
In-Reply-To: <42C9D47E.7090103@siunik.com>
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